1. Field of the Invention
The present invention relates to a scanning optical system and particularly to a scanning optical system for use in a printer, data symbol (e.g., bar code) reader, facsimile machine or other such apparatus. In particular, the present invention is related to a new and improved single element f.theta. lens unit for use in such a scanning optical system. Further, the present invention is related to a single element f.theta. lens unit with a reflective (i.e., mirror) surface with superior aberration compensation characteristics. The present invention is additionally related to a scanning optical system for use in a laser beam printer which can employ a single element refractive/reflective f.theta. optical element.
The present invention is also related to a laser scanning optical system including a reading optical system for reading information on an object surface and/or an image forming optical system for forming an image on a photoconductive surface.
The present invention is further related to a mirror surfaced f.theta. lens having at least one surface which is a rotationally asymmetrical aspherical surface, such as a two dimensional polynomial aspherical surface. With respect to a light beam incident on the mirrored surface f.theta. lens of the present invention, three optical surfaces are provided, one surface being a reflective optical surface and two surfaces comprising refractive optical surfaces. The refractive/reflective single element f.theta. lens of the present invention also serves to converge a divergent or parallel light beam incident thereon.
2. Background and Material Information
A conventional laser scanning optical system is provided with a light source such as a semiconductor laser, a light deflecting device such as a polygonal mirror, and a scanning lens system such as an f.theta. lens system. A laser beam is emitted by the light source, and is deflected by the light deflecting device. The deflected laser beam is then passed through the f.theta. lens system to scan a predetermined area on an object surface. Such scanning on the object surface by the deflected laser beam is referred to as scanning in the main scanning direction. While scanning in the main scanning direction is being carried out, the object surface is moved in a direction orthogonal to the direction of the main scanning, i.e., in an auxiliary scanning direction, and an auxiliary scanning is executed. Thus, the object surface is scanned two-dimensionally (i.e., in directions orthogonal with respect to each other).
Scanning optical systems generally include a plurality of lenses having complex shapes. In particular, such multiple element systems utilize anamorphic polished glass f.theta. lenses. Recently, to reduce the cost of the f.theta. lens system, plastic f.theta. lenses are used instead of glass f.theta. lenses.
A reflective type f.theta. element has also been utilized in an optical scanning system. Such reflective type f.theta. elements are shown, for example, in U.S. Pat. Nos. 5,572,353, 5,604,622 and 5,408,095. Each of these patents utilize an additional compensating optical element to compensate for various aberrations and to achieve acceptable quality. However, because the use of such a reflective element requires the utilization of an additional compensating lens to ensure that aberrations are adequately controlled, the complexity and size of the mechanism as well as the cost of the scanning system are increased.
Further, in these above-noted patents, an additional compensating element in the form of a toric lens is provided. However, since this lens is large, its cost is high and it would be advantageous, at least from a cost perspective, to eliminate such lens. However, if the toric lens is eliminated, it becomes very difficult to achieve the f.theta. function and to correct for curvature of field.
Other scanning systems are known which utilize an f.theta. element having opposing aspherical surfaces, one of which (the rear surface), is provided with a mirror. However, in such prior art, a polygonal mirror and an object surface (photosensitive drum) are located along an extention of the optical axis of the f.theta. element. As a result of this arrangement, the light emitted by the f.theta. element toward an object surface passes by the polygonal mirror, and the scanning optical system is very difficult to lay out. (i.e. to design)
It is well known that in the design of optical systems for, e.g., laser scanning units, various types of aberrations must be considered and controlled in order to ensure that adequate optical performance is provided. Some of these aberrations include f.theta. characteristic error, curvature of field and bowing of the scan line.
F.theta. characteristic (linearity error) is related to variations in the speed of the beam with respect to the surface of the scanning (photosensitive image forming) surface. Curvature of field is related to the defocus of a light beam with respect to the photosensitive surface. In other words, if the beam focus point is not precisely on the photosensitive layer, the beam diameter will increase and will result in a fuzzy image.
Bowing of the scan line is generated because of the fact that the f.theta. element, which can include a reflective surface, is tilted so that the beam reflected thereby passes out of the plane which includes the light beam emitted by the light source and the beam deflected by the light deflecting element. In other words, because of the tilting of the (reflective surface) f.theta. element, the beam reflected thereby forms a scanning line which is curved in the direction of the auxiliary scanning direction. In other words, a "bow" (i.e., a bend of the scanning line in the auxiliary scanning direction) occurs due to the tilted arrangement of the f.theta. element (reflective surface). Usually, to adequately eliminate these aberrations, as noted above, several lens elements are necessary in a scanning optical system.
Another approach to eliminating the effect of (or at least controlling) these aberrations has been to utilize what is known as a post objective type scanning optical system, employing a reflective f.theta. element formed as a concave mirror wherein the back side surface of the element is a reflective surface. A system of this sort is disclosed in U.S. Pat. No. 4,852,957. Both surfaces of the concave mirror utilized therein are formed as spherical surfaces and a glass plate of the mirror compensates for aberrations and a light converging means is located between the light source and the deflector.
As noted above, such prior art scanning systems utilize optical compensating elements, and it is a prime function of the present invention to eliminate the need for such additional compensating elements while still maintaining the optical characteristics of the f.theta. system. Thus, the present invention differs from the above-noted prior art by utilizing only a single optical element between the light deflecting member and the scanned surface while the prior art utilizes two elements and in utilizing more complex shapes, at least for a reflective face of the surface of the f.theta. element, i.e., shapes not having an axis of rotation.